Hitherto, electrically insulating tapes obtained by coating a tack adhesive on one surface of a polyethylene film or a polyvinyl chloride film and cutting such into a suitable width have been used as electrically insulating materials for coating electrical wires or splicing cables, etc.
These electrically insulating tapes are wound on an object to be coated. However, winding on the object to be coated cannot be always easily carried out, and there is a disadvantage that unevenness easily occurs on the insulating property of the coated parts due to differences in the skill of workers.
In order to minimize this disadvantage of electrically insulating tapes, electrically insulating tubes which do not require the tube to be wound when such is applied to the object to be coated, such as a polyethylene tube or a polyvinyl chloride tube, etc., have been used.
Recently, improvement of various types of properties including the heat resistance of the electrically insulating materials has been further required. However, this requirement cannot often be met with the above-described polyethylene tube or polyvinyl chloride tube.
Hence, polyimide resins have been of interest as electrically insulating materials satisfying the above-described requirement. However, a tube molding process involving extrusion molding which is used for other thermoplastic resins cannot be utilized for the polyimide resins because polyimide resins have a high melt viscosity and poor fluidity.
Further, since polyimide resins themselves do not have heat-fusible properties, it is not possible to utilize a process which comprises winding the film on a core material and thermally fusing such to make a tube.
Nevertheless, polyimide resins have extremely excellent properties including heat resistance as compared with polyethylene or polyvinyl chloride, but they cannot be widely used for industrial production of tubes because of difficulty in molding.